Covalent bond shortening and distortion induced by pressurization of thorium, uranium, and neptunium tetrakis aryloxides

Covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of the actinides, but the ionic versus covalent nature of metal-ligand bonding in actinide complexes remains controversial. The tetrakis 2,6-di-tert-butylphenoxide complexes of Th, U and Np form an isostructural series of crystal structures containing approximately tetrahedral MO4 cores. We show that up to 3 GPa the Th and U crystal structures show negative linear compressibility as the OMO angles distort. At 3 GPa the angles snap back to their original values, reverting to a tetrahedral geometry with an abrupt shortening of the M-O distances by up to 0.1 Å. The Np complex shows similar but smaller effects, transforming above 2.4 GPa. Electronic structure calculations associate the M-O bond shortening with a change in covalency resulting from increased contributions to the M-O bonding by the metal 6d and 5f orbitals, the combination promoting MO4 flexibility at little cost in energy.

with stirring at 60°C for 4 days. The volatiles were removed in vacuo to give a sticky white residue; this was then dried under vacuum at 120°C overnight to give the product as a free-flowing white powder (15.1 g, 27 mmol, 80% yield). 1.7 ZrBn4 (Bn = CH2Ph) 5 To a cooled stirred mixture of ZrCl4 (1.15 g, 5.0 mmol) in toluene (25 mL) was added benzyl magnesium chloride solution (25 mL, 1.0M in Et2O, 25mmol) at -78°C. The reaction mixture was covered in foil to protect against degradation by light and allowed to warm to room temperature overnight, after which it was stirred at room temperature for a further 24 hours. The volatiles were removed in vacuo and the remaining brown solid was extracted into toluene (25 mL) and concentrated to incipient crystallization (approx. 5 mL in volume). The liquors were stored at -20°C and yielded orange crystals of ZrBn4 after one week in 35% yield (0.8 g, 1.8 mmol). The 1 H-NMR spectrum in C6D6 agrees with the literature. 5

237
NpCl4 was prepared by chlorination of 237 NpO2 with a Cl2/CCl4/Ar gas mixture according to a modified literature method. 6,7 1.9 Synthesis of U(OAr)4 A 200 mL ampoule was charged with a stir bar, (N″)2U[κ 2 -(N,C)-N(SiMe3)SiMe2CH2] (0.589 g, 0.820 mmol), HOAr (0.675 g, 3.28 mmol), and toluene (20 mL). The solution was heated at 110 °C for 12 h, after which the solvent was removed in vacuo to give U(OAr)4 as a dark yellow solid (0.611 g, 0.577 mmol, 70%). 1 H NMR spectrum collected in C6D6 were consistent with the data previously reported for complex U(OAr)4. 8 Single crystals suitable for X-ray diffraction could be obtained by crystallization from a saturated toluene solution at -35 °C.

Synthesis of Th(OAr)4
A 250-mL sidearm flask equipped with a stir bar was charged with ThCl4(DME)2 (0.83 g, 1.50 mmol) and THF (15 mL). A THF (10 mL) solution of KOAr (1.47 g, 6.02 mmol) was added dropwise at room temperature with stirring and the reaction mixture stirred for 12 h. After isolation via cannula filtration and drying in vacuo, Th(OAr)4 was isolated as a colourless solid (1.35 g, 1.28 mmol, 85%).The 1 H NMR spectrum collected in C6D6 is consistent with the data previously reported for Th(OAr)4. 8 Single crystals suitable for X-ray diffraction could be obtained by crystallization from the slow diffusion of hexane into a THF solution of Th(OAr)4.

Synthesis of Np(OAr)4
A THF solution of KOAr (2.3 ml, 0.162 M) was added to 1 ml THF solution of NpCl4 (38.7 mg, 0.102 mmol), leading to an immediate colour change of the reaction mixture to deep red. The reaction mixture was allowed to stir for further overnight and the solvent stripped under reduced pressure. Then 10 ml toluene was added and the red solution was isolated by syringe filtration (PTFE membrane, 0.45 µm) and concentrated under reduce pressure to ca. 1 mL to afford red crystals of the target product Np(OAr)4 (94.4 mg, 0.0892 mmol, 87.8%).

Reaction to target Ce(OAr)4
To Ce(N i Pr2)4 (8 mg, 0.015 mmol) in C6D6 (0.2 mL) was added a solution of HOAr (12.2 mg, 0.059 mmol) in C6D6 (0.2 mL) at room temperature. A colour change from purple to brown was observed over 16 hours. The reaction was followed by 1 H-NMR (500.1 MHz, C6D6, 300 K) over 6 days; resonances corresponding to those reported for Ce(OAr)3 were observed to appear and grow in over this period, 9 along with resonances presumed to correspond to other Ce(III) containing species (see Supplementary Figures 3 and 4), as these also appear in the paramagnetic region of the spectrum, and resonances corresponding to HN i Pr2. 10 The apparent reduction of Ce(IV) to Ce(III) is presumed to occur via Ce-X bond homolysis (X= O or N). it is therefore not possible to prepare Ce(OAr)4 directly by this method, and given the presence of elongated U-O bonds in the U IV analogue, we did not pursue routes including an additional oxidant. 11 Raw 1 H-NMR data are available at dx.doi.org/10.17632/2mhj4xy8d4.1. The SAMBVCA applet 2.0 was used to estimate the volume each OAr ligand occupies around the metal centre within a sphere of 3.5 Å radius, centred on the metal centre (%V_Bur). 12 All default parameters in the SAMBVCA applet were used as found, unless otherwise stated. The coordinates used for the OAr ligand were taken from CCDC structure 1185610 and are given below in Supplementary Table 1. 13 Approximations used include the imposition of a 180° M-O-C bond angle (which reduces the volume required by the OAr ligand) for all metal complexes and using ligand coordinates for a single OAr ligand from U(OAr)4 CSD structure 1185610, without optimising its geometry for the other metal centres. Approximations are discussed below. To obtain the %V_Bur of a single OAr ligand the following steps were followed for each metal(IV) ion; 1) The OAr.xyz file was loaded into the applet.
2) O(0) was set as the centre of the sphere (i.e. the metal atom).
3) The z-axis was defined by clicking on C(1). (This will artificially set the M-O-C angle to 180°). 4) The xz-plane was defined by clicking on C(2) and C(6). 5) The Bondi van der Waals radii were scaled by 1.17 (applet default). 6) The sphere radius was set to 3.5 Å (applet default). 7) The distance of the coordination point from the centre of the sphere was set to the average for tetracoordinated M-OAr complexes. (This was determined from CSD database v5.40 (+3 updates) and Mogul geometry search of these databases; Zr-OAr 1.913 Å, Ce-OAr 2.119 Å, Th-OAr 2.192 Å, U-OAr 2.119 Å). 8) The mesh spacing for numerical integration was set to 0.10 (applet default) and the job was then submitted to the applet. 9) The %V_Bur for MOAr4 was then estimated by multiplying the calculated %V_Bur for one OAr by four.
This was deemed a reasonable approximation as the observed structures for U(OAr)4 and Th(OAr)4 have I-4 symmetry at ambient pressure. Given the ease of preparation of U(OAr)4 and the similar ionic radii of cerium(IV) and uranium(IV), at first glance it seemed that it should be possible to prepare Ce(OAr)4 but the reduction potentials of Ce(IV) versus U(IV) explain why Ce(OAr)3 is observed (Supplementary Table 4). The U(OAr)4 complex is particularly crowded, resulting in U-O bond lengths longer than typically found for tetra-aryloxide complexes of uranium(IV). In the product, or presumed reaction intermediates Ce(N i Pr2)x(OAr3)4-x, this crowding is relieved by reductive Ce-X bond homolysis (X = N or O), resulting in the Ce(III)OAr3. N.b. it is unclear whether it is . N i Pr2 or . OAr that is eliminated by bond homolysis as the expected coupled products are not observed in 1 H-NMR spectra and products arising from H abstraction from solvent would be indistinguishable (we do not observe the H in HN i Pr2). For reference the %VBur of N i Pr2 is 21.9 % (Ce(N i Pr2)4 total %V_bur = 87.6 %).
Supplementary Table 4: Reduction potentials of U(IV) and Ce(IV) with respect to a standard hydrogen electrode.

E°/V (with respect to standard hydrogen
Zirconium(IV) has a significantly smaller ionic radius than cerium(IV) or uranium (IV), with typical Zr(IV) tetraaryloxide Zr-O bond lengths of 1.913 Å. On the basis of the estimated %V_Bur of 109.6% for Zr(OAr)4 we do not expect it to be possible to form Zr(OAr)4, which is in line with our own, and others', experience. 15

Crystal Structure Determinations
Single crystals of each material were loaded into Merrill-Bassett diamond anvil cells 16,17 with a chip of ruby to enable the pressure to be measured from its fluorescence wavelength. Hydrostatic media were fluorinert FC70 (0-1 GPa), pentane-isopentane (1-5 GPa) and Daphne oil (0-2.5 GPa). For the Th and U derivatives data were collected on beamline I19 at Diamond Light Source using radiation of wavelength 0.4859 Å. Additional data for Th(OAr)4 at ambient pressure, 2.83, 3.16 and 4.30 GPa and for U(OAr)4 at ambient pressure, 0.44, 1.84 and 2.67 GPa and all data for Np(OAr)4 were measured using Mo Kα radiation on Bruker Apex diffractometers with sealed-tube sources. Data were collected to maximum pressures of 4.3, 3.9 and 4.1 GPa for the Th, U and Np systems.
Increasing the pressure from 2.88 to 3.03 GPa for Th(OAr)4 and from 2.88 to 3.02 GPa for U(OAr)4 led to a degradation in the quality of the diffraction data as a result of the phase transitions at 3 GPa and possibly partial amorphisation, Supplementary Figure 5 showing the variation of Rint with resolution for both complexes at these pressures. Although the peaks showed some broadening, there was no sign of splitting of peaks, which might indicate a reduction in symmetry and twinning, but there was a marked decrease in the intensity of the high-resolution data (Supplementary Figure 6). Similar comments can be applied to the data for U(OAr)4 at 2.88 and 3.02 GPa. The decline in crystal quality led to increases in the uncertainties of the structural parameters at the highest pressures achieved in this study. Crystal structures were solved using SHELXT 18 and refined against |F| 2 . Distances and angles within the ligands restrained in the all the refinements against high-pressure data to ambient pressure values; enhanced rigid body restraints were also applied to the anisotropic displacement parameters. Equation of state analysis was carried out with EoSFIT. 19 Crystallographic information files are available in the Cambridge Database with deposition numbers 1981193-1981195, 1981197-1981201, 1981204-1981216 and 2095929-2095947. Crystal and refinement data are also listed in Supplementary Tables 6, 7 and 8.

Projection analysis of crystallographic data
In order to identify the structural changes that give rise to the variation in unit cell dimensions, The length (or 'height') of the ThO4 tetrahedron in Th(OAr)4 projected onto c at ambient pressure.
The cell dimensions are a = 14.0696 (2), c = 13.6511(2) Å and the coordinates of the Th, O1 and C1 atoms are: The contribution of the remainder of the structure is obtained in this example from the difference of these two measurements with the length of the c axis: 13.6511(2) -9.867(22) = 3.784 (22).
A similar calculation can be carried out along the a axis using the midpoint of O1 and O1b to define the dimension of the ThO4 tetrahedron (see Fig. 1 in the main paper, where O1b is generated by the operation ½ − y, ½ + x, ½ − z). The advantage of using the midpoint is that the breakdown of contributions is the same along the a and b axes. The dimension of the Th(C1)4 and thereby the 'link' contribution, is likewise defined using the midpoint between C1 and C1b.
Plots showing the variation of the projected dimensions with pressure are shown in Supplementary Figure 9.
The important point is that collapse in the c axis length and the slight increase in the a axis length at 3 GPa for Th(OAr)4 and U(OAr)4 arises from the discontinuous changes in the dimensions of ThO4 and UO4 tetrahedra projected along c and a, respectively.

Shape index calculations
The continuous symmetry measure, or shape index, is a parameter devised originally by Pinksy and Avnir 20 to assess the extent of distortion of a given polyhedron from an ideal reference polyhedron. 21 In the context of the present study, the parameter was used to measure how far the MO4 moiety in M(OAr)4 (M = Th, U or Np) deviated from an ideal tetrahedron as a function of pressure; a shape index of zero corresponds to an ideal tetrahedron.
The atomic coordinates of the M and O atoms contained in the cifs (see above) were converted from fractional to Cartesian using Mercury and then used as input to the program SHAPE. 22 The following is a fragment of the input file for Th(OAr)4, where the geometries of the ThO4 unit at ambient pressure and 4.30 GPa is compared with both a tetrahedron and a square; see Example 1 in the SHAPE manual for further explanatory notes. The shape index for the complex at 0 GPa is 0.064 relative to a tetrahedron and 32.180 relative to a square, meaning that it closely resembles a tetrahedron, but is very unlike a square. Standard